Often it is desirable to separate a mixture of nitrogen and methane into nitrogen-rich and methane-rich components. One such situation is when a stream from an oil or gas reservoir contains nitrogen in other than insignificant concentrations. The nitrogen could be naturally occurring and/or could have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. Generally the stream from the reservoir will undergo initial processing wherein heavier components, such as natural gas liquids (NGL), are removed and then the remaining stream containing primarily nitrogen and methane is separated cryogenically in one or more rectification columns. When a single rectification column is used to make the cryogenic separation, the column is often driven by a recirculating fluid heat pump. A recent significant advancement in such a process is described in U.S. Pat. No. 4,501,600--Pahade.
An effective cryogenic separation process requires refrigeration to carry out the separation and to compensate for thermal inefficiencies such as ambient heat leak into the cold equipment. Further, the available refrigeration must be at the proper temperature levels in order to maintain the cold temperatures required for the cryogenic separation process. Of course, refrigeration may be provided to a process from an external source but this is costly.
Process refrigeration may be generated internally by the pressure level reduction or expansion of incoming feed or outgoing methane or nitrogen but such a procedure may have limited usefulness. For example, it may be desirable to keep the feed stream pressure relatively high in order to reduce equipment sizes or to maintain desired process conditions such as column temperature levels. Methane product may be desired at elevated pressure in order to keep pumping to pipeline pressure requirements low. Nitrogen may be required at elevated pressure to facilitate injection into the petroleum reservoir for EGR or EOR operations. Thus it may be desired that no expansion, or only a limited amount of expansion, of the feed, methane or nitrogen streams be carried out.
As mentioned, a single column cryogenic rectification process is often driven by a recirculating fluid heat pump. Such an arrangement does not add refrigeration to the column but rather transfers refrigeration within the column. It would be desirable to have a cryogenic separation process employing a recirculating fluid heat pump wherein added refrigeration is supplied to the column at needed temperature levels without need for significant amounts of outside added refrigeration or large expansion of process streams.
It is therefore an object of this invention to provide an improved process for the separation of methane and nitrogen.
It is a further object of this invention to provide an improved single column cryogenic distillation process for the separation of methane and nitrogen driven by a recirculating fluid heat pump wherein added refrigeration is provided to the process at the required temperature levels without need for significant amounts of outside added refrigeration or large expansion of process streams.